Stage 1 fire restrictions, enacted for unincorporated areas of western Boulder County.

Radon-Resistant Construction
Hispanic family building a new home

Radon-Resistant Construction

Radon-Resistant Construction

Radon-Resistant Construction is the installation of a “passive” radon mitigation system during the construction of a new single- or multi-family home. A “passive” system does not include a radon mitigation fan. When properly installed these systems can lower indoor radon levels by about 50%. Some “passive” systems may require the addition of a fan if radon levels are elevated. For example, in a home with a radon level of 10.0 pCi/L the radon would only be lowered to 5.0 pCi/L by a properly functioning “passive” system.

Construction Techniques

graphic explaining how radon enters a home

Radon-Resistant Construction techniques may vary, depending on the foundation and site requirements. A building constructed to radon-resistant standards will include these basic elements:

A. Gas-Permeable Layer – a layer placed beneath the slab or flooring system allows the soil gas to move freely beneath the building. In many cases, the material used is a 4-inch layer of clean gravel.

B. Plastic Sheeting – plastic sheeting placed on top of the gas-permeable layer and under the slab helps prevent the soil gas from entering the building. In crawlspaces, the sheeting is placed over the crawlspace floor.

C. Sealing & Caulking – sealed openings in the concrete foundation floor reduce soil gas entry into the building.

D. Vent Pipe – a 3- or 4-inch gas-tight or PVC pipe (commonly used for plumbing) running from the gas-permeable layer through the building extending above the roof ensures that radon and other soil gases are safely ventilated above the building.

E. Junction Box – an electrical junction box is installed in case a fan is needed later to reduce building radon levels below 4.0pCi/L.

Radon-resistant construction techniques can not only reduce radon levels, but can also decrease soil moisture. This helps reduce mold, mildew, and odors, especially when a “passive” system is activated with a radon reduction fan.

Local Requirements

Currently in Boulder County, new single-family homes must be built with Radon-Resistant Construction in the following municipalities:

  • The City of Boulder
  • The City of Longmont
  • The City of Louisville
  • The Town of Erie
  • The Town of Superior
  • Unincorporated Boulder County

Testing After Occupancy

A new home constructed with Radon-Resistant Construction should be tested for radon as soon as possible after move-in. This is to ensure the “passive” system is lowering the radon levels in the building below 4.0 pCi/L. If testing shows a level of 4.0 pCi/L or higher, a fan can be added to the system to reduce the radon levels. After the addition of a fan, the home will need to be retested to make sure the fan is operating properly and lowering the radon levels below 4.0 pCi/L.

Contact your building department for information about radon test kits for new homes built with Radon-Resistant Construction.

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